Rare earth element-containing alloys composed so as to form the RE.sub.2 TM.sub.14 B tetragonal crystal phase have been melt spun under carefully controlled processing to produce useful permanent magnet materials as disclosed in Croat U.S. Pat. No. 4,802,931 and U.S. Pat. No. 4,851,058. Such melt-spun materials either as quenched or in an overquenched and annealed condition consist essentially and predominantly of the tetragonal crystal, prototype Nd.sub.2 Fe.sub.14 B phase. The tetragonal crystal-containing grains are very small, typically less than a few hundred nanometers on the average in grain size, and are surrounded by one or more secondary grain boundary phases which contribute to the permanent magnet characteristics of the composition. This fine grain material is magnetically isotropic, and the melt-spun ribbon fragments can be pulverized to a suitable powder, combined with a suitable binder material and molded into useful bonded permanent magnets as disclosed in Lee et. al. U.S. Pat. No. 4,902,361.
Where permanent magnets of higher energy product are desired, it is known that the melt-spun powder material can be hot pressed to form a fully densified permanent magnet body and that, where desired, such fully densified body can be further hot work deformed. These practices are disclosed, for example, in Lee U.S. Pat. No. 4,792,367 and 4,844,754.
The fine grained, melt-spun, rare earth element-containing material is initially in the form of ribbon particles or a powder produced by comminution of the ribbon fragments. In order to hot press or otherwise hot work the material, it is necessary that it be heated to a suitable hot working temperature typically in the range of 700.degree. C. to 800.degree. C. As disclosed by Lee, it is prudent to heat the powder in vacuum or suitable inert gas that provides a dry and substantially oxygen-free environment in order to prevent the powder from burning. In attempting to work with such readily oxidizable rare earth element-containing materials, it has been necessary to provide a suitable protective atmosphere in which the rare earth and other constituents are not oxidized and the permanent magnetic properties of the materials are not degraded.
In powder metallurgy practices, it is known to produce a compact by pressing suitably ductile powder particles together at ambient conditions. This can be done (and is done with larger grain size rare earth element-transition metal-boron, RE-TM-B, materials in sintering processes) to produce a partly densified, porous body in air at room temperature. However, if such a Nd.sub.2 Fe.sub.14 B compact is heated preparatory to hot working, it must be protected from oxidation in order to avoid degradation of the permanent magnet properties. Obviously, it is possible to enclose the operative portion of a press in such a non-oxidizing atmosphere, but it is expensive and impractical to adapt such apparatus for high speed production if accurate powder feeding, powder heating, compaction and hot working are to be carried out entirely within such a special atmosphere chamber. Such a press would be very expensive to construct and operate and cumbersome to operate and maintain.
Accordingly, it is necessary to develop a practice for hot pressing and, optionally, the additional hot work deformation of such rare earth element-containing powder alloy materials so that the rapid and efficient production of permanent magnets can be accomplished. It is a general object of this invention to provide a method for hot pressing and additional hot work processing of RE-TM-B type powder materials on relatively inexpensive open air-type presses in a way that provides a suitable protection from oxidation or burning.